1. Field of the Invention
The present invention relates to a fuel injection method for a direct-injection auto-ignition internal-combustion engine preferably of diesel type. More particularly the invention preferably relates to diesel type engines operated in a low-temperature combustion mode of the fuel mixture, more commonly known as LTC (Low-Temperature Combustion).
2. Description of the Prior Art
This LTC combustion type notably affords the advantage of limiting the production of pollutants due to combustion and in particular of limiting the production of nitrogen oxides (NOx) and of particles.
This combustion is generally carried out using a large amount of exhaust gas recirculated (EGR) to the engine intake to lower the combustion temperatures, allowing reduction of NOx emissions. Use of a high proportion of pre-mix air and fuel provides a relatively homogeneous mixture before the combustion, which limits the production of particles to be limited.
However, this combustion type has the major drawback of producing high HC emission levels that are discharged to the engine exhaust. This HC production is essentially linked with the low combustion temperatures that do not allow total oxidation of the HC to CO, then to CO2.
Patent application CN-1,434,193 describes an engine with a cylinder within which a piston carrying a hollow bowl slides in a reciprocating rectilinear motion. A combustion chamber intended for a fuel mixture is thus defined by the cylinder wall, the face of the cylinder head opposite the piston, the upper face of the piston and the walls of the bowl.
As explained in detail in the aforementioned document, the thermal losses (heat losses generated upon combustion of this fuel mixture) through the walls of the combustion chamber of the engine are to be minimized. The surfaces of the bowl are therefore coated at least partly with a thermal insulation coating, of ceramic type for example having a very low thermal conductivity.
This coating thus limits the heat loss generated upon combustion of the fuel mixture, which allows maintaining high combustion temperatures. These high temperature levels thus promote better oxidation of the unburnt hydrocarbons (HC) and of the carbon monoxides (CO). Furthermore, using such a coating causes a rise in the temperatures of the combustion chamber walls, which has the effect of limiting the formation of liquid fuel films that form notably at the surface of the piston. In fact, these liquid fuel films are an insignificant source of HC production, in particular in the LTC combustion type.
The temperature increase however leads to drawbacks harmful to the combustion, such as a reduction of the fuel mixture auto-ignition delay and an increase in the combustion velocity. These excessive combustion velocities, which generally lead to high energy release peaks, are bad because they are combined with high combustion noise levels (or engine noise).
The present invention overcomes the aforementioned drawbacks by a fuel injection method that allows the combustion noise levels to be reduced.